Add pair-kerning scaffolding and sidebearing scale

[terryspitz]

Summary

Stacked on top of #98 (Proofs tab). Adds a minimal kerning pipeline so the
proof snapshots show the kerning diff as a reviewable visual change, both on
the SVG-rendered tabs and on the proofs tab (which uses a CSS web font).

Changes

• src/generator/Spacing.fs (new) — curated override map for notorious
  pairs (AV, AT, To, Ty, Va, Vo, Wa, Ya, LT, fi, fl, rn, …) plus
  open-pairs for slab sequences (din, Num, NN, MM, UU) and targeted
  fixes for l/j (more room left of l, less right of l, tighter
  left of j against non-descenders). pairKernInt looks up the table;
  pairKern returns it as a float in glyph units.
• src/generator/Axes.fs — one new field:
  • sidebearingScale — multiplier on the existing thickness + serif
    sidebearing padding; lets the whole font be tightened or opened
    uniformly without retuning tracking. Default 1.0 keeps output
    bit-identical.
• src/generator/Font.fs —
  • width multiplies the sidebearing term by sidebearingScale.
  • New pairKern / pairKerns for per-pair and per-string kerns.
  • stringWidth now includes kerns so measured width matches rendered
    advance.
  • stringToSvgLineInternal applies the kern to the second glyph of
    each pair. Every glyph except the first is kerned against its
    predecessor; the first glyph has no predecessor so it lands at
    offsetX unshifted.
• src/explorer/Api.fs — generateFontGlyphData now also returns
  kerningPairs: { left, right, value }[] so the web font can carry
  the kerning.
• web/src/fontExport.js — feeds those pairs into opentype.js as
  font.kerningPairs, keyed by glyph index. opentype.js emits a kern
  table that modern browsers honour for CSS-rendered text, so the
  proofs tab (which uses @font-face) picks up the kerning.
• src/generator/tests/FontTest.fs — conservation-law tests:
  stringWidth = Σ advances + Σ kerns; unknown pairs return 0;
  italic axis does not change pair kerns (overrides live in the
  pre-shear frame); sidebearingScale scales the per-char sidebearing
  linearly.

Test plan

• dotnet test src/generator/tests/ passes (new conservation tests
  green).
• proofs-uppercase and proofs-lowercase Playwright snapshots
  diff cleanly — diff is tightened / opened spacing on seeded pairs
  (AV, To, rn, fi, din, *l, *j, …), no glyph corruption or
  overlap.
• Re-baseline the two proof snapshots on this branch after visual
  review.
• italic=0 and italic=0.3 on the Proofs tab render balanced
  spacing on AVATAR, To, fi, rn.

https://claude.ai/code/session_01NSaWc8pxLiqZYTpVodbciX